P-215 - The Role of Microbe-derived Nucleoside Infochemical dAdo in Plant Immunity and Cell Death

The colonization of plant roots by the beneficial fungal endophyte Serendipita indica follows a biphasic strategy, transitioning from an initial biotrophic phase to a cell death-associated phase. Using multi-omics analyses, we identified two apoplastic fungal effectors, SiNucA and SiE5NT, which synergistically produce the nucleoside metabolite 2′-deoxyadenosine (dAdo). dAdo acts as an inducer of regulated cell death (RCD) and elicits specific transcriptional and metabolic responses in host cells. Uptake of dAdo into root cells is mediated by the equilibrative nucleoside transporter AtENT3, with AtENT3 loss-of-function mutants showing resistance to dAdo- and fungal-induced RCD, thereby impairing fungal colonization. This establishes a direct link between purine metabolism and cell death regulation in plants. Unlike in animals, where dAdo-induced cell death is caspase-dependent, plants lack caspases, necessitating a genetic screen to identify key regulators. This revealed an uncharacterized TIR-NLR protein, ISI, which is expressed in roots and induced by S. indica. A mutant A. thaliana line lacking ISI exhibit partial resistance to dAdo but display root autoimmunity, connecting the purine metabolism to immune signaling. These findings uncover a novel immunometabolic pathway that regulates root-microbe interactions, expanding the role of purine-based signaling molecules and TIR-NLR proteins beyond pathogen defense in leaves to beneficial fungal colonization in roots.